A nuclear reactor produces electrical power by heating water in a reactor pressure vessel that contains a nuclear fuel core in order to generate steam which is used in turn to drive a steam turbine. The reactor pressure vessel includes a cylinder surrounding the nuclear fuel core. This cylinder is the core shroud. Feed water is admitted into the reactor pressure vessel and flows through an annular region which is formed between the reactor pressure vessel and the core shroud. Within the annular region, jet pump assemblies are circumferentially distributed around the core shroud. The core shroud includes various welds which are later discussed in detail herein. A core shroud head is positioned atop the core shroud. The material of the core shroud and associated welds is austenitic stainless steel having reduced carbon content. The heat-affected zones of the shroud welds have residual weld stresses. Therefore, the mechanisms are present for the shroud welds to be susceptible to stress corrosion cracking.
Stress corrosion cracking in the heat affected zone of any shroud weld diminishes the structural integrity of the shroud. In particular, a cracked shroud increases the risks posed by a Loss-of-Coolant Accident (LOCA) or seismic loads. During a LOCA, the loss of coolant from the reactor pressure vessel produces a loss of pressure above the shroud head and an increase in pressure inside the shroud, i.e., underneath the shroud head. The result is an increased lifting force on the shroud head and on the upper portions of the shroud to which the shroud head is bolted. If the core shroud has fully cracked girth welds, the lifting forces produced during a LOCA could cause the shroud to separate along the areas of cracking, producing undesirable leaking of reactor coolant. Also, if the shroud weld zones fail due to stress corrosion cracking, there is a risk of misalignment from seismic loads and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.
Thus, the core shroud is examined periodically to determine its structural integrity and the need for repair. Ultrasonic inspection is a known technique for detecting cracks in nuclear reactor components. The inspection area of primary interest is the outside surface of the core shroud and the horizontal mid-shroud attachment welds. However, the core shroud is difficult to access. Installation access is limited to the annular space between the outside of the shroud and the inside of the reactor pressure vessel, between adjacent jet pumps. Scanning operation access is additionally restricted within the narrow space between the shroud and jet pumps. The inspection areas are highly radioactive, and are located under water 50 to 80 feet below the operator's work platform. Thus, inspection of the core shroud in operational nuclear reactors requires a robotic device which can be installed remotely and operated within a narrowly restricted space.
Remote operation is mandatory due to safety risks associated with radiation in the reactor. During reactor shutdown, servicing of components requires installation of inspection manipulators or devices 30 to 100 feet deep within reactor coolant. The inspection equipment typically consists of manually controlled poles and ropes to manipulate servicing devices and/or positioning of these devices. Relatively long durations are required to install or remove manipulators and can impact the plant shutdown duration. In addition, different inspection scopes can require several manipulator reconfigurations requiring additional manipulator installations and removals. The long durations cannot only impact plant shutdown durations, but also increase personnel radiation and contamination exposure.
Plant utilities have a desire to reduce the number of manipulator installations and removals to reduce radiological exposure as well as cost and plant outage impact. This invention allows the number of reconfigurations, installations and removals to be minimized. In addition, plant utilities have relatively small working areas near the access point of the reactor cavity. Therefore, the size of the manipulators can impact other activities during plant shutdown. The design of this invention allows the elimination of large track ring typically utilized on similar equipment. Plant utilities also desire flexible and effective coverage on the reactor core shroud. This invention allows the manipulator operations to position end effectors in various locations on the shroud which are often inaccessible to currently designed tooling. The small profile and flexible axes system of this invention provide efficient core shroud coverage which can be significantly greater than the coverage provided by currently existing equipment.